The invention is based on a vehicle power brake system including a motor driven hydraulic pump.
A motor vehicle brake system of this kind is described in DE 195 48 207. This motor vehicle brake system has a hydraulic pump that can be driven with an electrical pump motor to build up a brake fluid pressure for power braking. The hydraulic pump aspirates from a brake fluid reservoir. A pressure side of the hydraulic pump is connected to wheel brake cylinders, each of which is preceded by a solenoid inlet valve that is closed in its normal position. Furthermore, each wheel brake cylinder is followed by a solenoid outlet valve that is open in its normal position and is connected to the brake fluid reservoir. The solenoid valves are triggered by an electronic control device, which receives an input signal from a for example pedal-actuated brake power set-point transmitter and from brake pressure sensors connected to the wheel brake cylinders.
To increase pressure in the wheel brake cylinders, the outlet valve is closed and the inlet valve is opened until a brake pressure is adjusted in the wheel brake cylinders that is a function of the signal of the brake power set-point transmitter. To reduce the pressure in the wheel brake cylinders, the outlet valve is opened.
In addition to its function for adjusting the brake pressure in the wheel brake cylinders, the inlet valve has a safety function: if the brake fluid pressure exceeds a permissible maximum pressure due to a malfunction of the electronic control device or of a pump pressure sensor connected to the pressure side of the hydraulic pump, then the inlet valve, which closes by means of a spring force, opens so that brake fluid flows from the pressure side of the hydraulic pump, through the inlet valve and the outlet valve, which is open in its normal position, back into the brake fluid reservoir so that the maximum pressure in the motor vehicle brake system is limited. No brake pressure is built up in the wheel brake cylinder.